Immobilization of living organisms is a phenomenon that occurs naturally when cells grow on surfaces. Various studies have shown differences in fermentation characteristics of free and immobilized cells. Immobilization of living microbial whole cells influences cell growth rate and morphology and sometimes alters metabolic behaviour. Furthermore, changes of tolerance of cells to certain environmental stress factors have in some cases been observed for specific systems.
Cachon et al. (1998) described differences in cell physiology during batch cultures with and without pH control and continuous cultures with free and immobilized Lactococcus lactis, depending on the culture mode. The redox states, enzymatic pool and intracellular pH are specific for immobilized cells and differ from those of free cell cultures.
An increased tolerance to product inhibition of immobilized cells have in some cases been observed. Fortin and Vuillemard (1989) have shown that protease production by Myxococcus xanthus is inhibited by end-products of proteolysis, immobilized cells being less sensitive to this inhibition.
A decreased sensitivity to the adverse effects of the product, butyric acid, at high Clostridium butyricum cell densities was observed by Teixeira de Mattos et al. (1994). After prolonged culture with free cells, the cells acquired the capacity to form aggregates spontaneously. Furthermore, the aggregated cells evolved with fermentation time. The same study showed a metabolic change (stimulation of lactic acid production) for Lactobacillus laevolacticus at high cell densities or when lactic acid was added in the medium, which was explained by the role of lactic acid as a positive effector for lactic acid production by this strain.
Immobilized (by entrapment in Ca-alginate or by adsorption on pre-formed cellulose beads) Saccharomyces cerevisiae and Acetobacter aceti showed increased tolerance to ethanol and acetic acid, respectively, compared to free cells (Krisch and Szajani 1997). Cells released from the gel beads or solid supports showed an intermediate tolerance compared to free and immobilized cells.
Further, a modification in the lipid-protein ratio of the membranes have been observed for immobilized cells (Keweloh et al. 1989; Keweloh et al. 1990). The proportion of saturated fatty acids was higher in immobilized cells.
The immobilization of various Lactococcus and Leuconostoc strains reduced the inhibitory effect of quaternary ammonium sanitizers (QAS) on these strains (Trauth et al. 2001).
Escherichia coli cells immobilized in agar gel layers showed increased resistance to an antibiotic (latamoxef) as compared to free (planktonic) cells (Jouenne et al. 1994).
During chemostat culture with cells of Lactobacillus plantarum immobilized on chitosan treated polypropylene matrix, a shift in the metabolic pathway from homofermentative to heterofermentative has been observed (Krishnan et al. 2001).
Although the use of immobilized cell cultures have been described with respect to certain systems, there remains a need for its study in other systems to identify other potential useful applications of this technology.